•±9± G. H. Williams — Volcanic Rocks of South Mountain 



sandstone lies flat or in synclinals ; and the sections made by 

 Miss Bascom across Monterey Peak, Pine Mountain, Jack's 

 Mountain, and Haycock near Monterey, all indicate that the 

 sandstone is altogether above the volcanic rocks, and that it 

 has been only sporadically left by erosion on the east side of 

 the mountain in Pennsylvania. In Maryland the . volcanic 

 rocks are flanked both on the east and west by sandstone (see 

 map). ]STo alternations of relatively thin beds of sandstone 

 and lava have thus far been observed. The contacts of the 

 sandstone above the porphyry on the old tapeworm railroad 

 southwest of Maria's Furnace, and above the greenstone in the 

 Jack's Mountain railroad tunnel are both admirable exposures, 

 but both seem to be thrust-planes and are not contacts of original 

 deposition. 



The South Mountain volcanic rocks therefore become, not 

 merely in their petrographical character and richness in metal- 

 lic copper, but also in their stratigraphical position, comparable 

 with the Keewenawan or I^ipigon series of Lake Superior. 



5. Chemical Alteration and Metamorphism. — Extensive 

 chemical changes, involving devitrification and the formation 

 of new minerals, have gone on in all the volcanic rocks of South 

 Mountain without destroying the original structures. In other 

 cases, where there has been movement and shearing, the same 

 rocks have lost both their original minerals and structures by 

 a process of complete metamorphism. The results are more or 

 less perfectly foliated schists and slates, whose origin can be 

 positively traced to the volcanic rocks, and whose present form 

 can be shown to depend upon the intensity of the dislocation 

 to which they have been subjected. 



The chemical changes which have not affected the massive 

 character of the rocks consist of the formation of new minerals 

 to accord with the altered physical conditions. These have, as 

 a rule, merely replaced the former minerals so as to leave the 

 original structure of the rock intact. In the basic rocks the 

 new minerals are epidote, fibrous green hornblende, chlorite, 

 serpentine, iron oxide, and, to less extent, calcite and quartz. 

 Of these by far the most important is epidote. Indeed the 

 conditions for the formation of this substance must have been 

 exceptionally favorable, as it has everywhere been produced in 

 great abundance. Some of the. finer material, like volcanic ash 

 or breccia cement, is wholly altered to this mineral. It is 

 also the most common filling of the amygdules. 



In the acid rocks there has been a complete recrystallization 

 of all glass into a fine quartz-feldspar mosaic, which, however, 

 still exhibits the original structures. The conditions favorable 

 to epidote formation are manifest in these rocks in the presence 

 of large amounts of the manganese epidote (piedmontite). 



